494
A. A. MOSCONA
one might assume that such space would be taken up by an extracellular material which represents a cellular product coating the
migrating cells. Faure-Fremiet and Wallich (1925) demonstrated that
such materials may expand beyond the cell-body and be deposited as
trails. As discussed elsewhere (Moscona, 1960), the characteristics of
such cellular deposits appear to differ according to treatment of the
cells, variations in medium composition, pH, ions, temperature. In
general, the deposits are more noticeable in low-protein media (see also
Rosenberg, 1960; A. C. Taylor, 1961; L. Weiss, 1961), but whether this
is due to increase in quantity or to a changed colloidal state is unknown.
The possibility was considered that tracks of such extracellular material
might provide guide-lines for migrating cells, thus imposing on an
aggregating system a certain degree of non-randomness. The actual
deposition by migrating cells of tracks that were followed by other cells
was clearly visualized in time-lapse cine-films (Pomerat, 1959; unpublished films by this reviewer; similar effects are discussed by Gustafson,
1961). The question was raised whether the composition or the physicochemical properties of such extracellular materials might not provide
"information" transferable to cells coming in contact with it, and thus
play a role in "self-recognition" effects. Similar speculative possibilities
were recently considered by L. Weiss (1961). Okada (1959, 1960) found
that the colloidal material appearing in aggregating cell cultures took
up S
35 and that the label was incorporated in the matrix of the aggregates. Functional similarities may thus exist between the extra-cellular
materials of dissociated cells appearing in the course of aggregation and
the intercellular substances of restituted tissues (Moscona, 1960, 1962b).
As stated above, self-aggregation does not lend itself readily to
rigorous control; the results, complicated by secondary effects, are
variable and difficult to assess quantitatively. The use of hanging drops,
in which the cells sink to the interface between the liquid and the air
(Grover, 1962), reduces some of the difficulties but introduces others.
Self-aggregation is therefore useful primarily in descriptive work on
histogenesis of aggregates, and in studies on cell movement in relation
to various substrata or interfaces. For more precise studies on the
dynamics of cell contact and histotypic association, experimental
systems are to be preferred in which the cells are brought together
independently of their individual mobilities, under conditions amenable
to control and measurement.
C. MEDIATED AGGREGATION
1. Centrifugation
Trinkaus and Groves (1955) and Weiss and James (1955) used
centrifugation to make pellets of trypsin-dissociated cells which,
A. A. MOSCONA
one might assume that such space would be taken up by an extracellular material which represents a cellular product coating the
migrating cells. Faure-Fremiet and Wallich (1925) demonstrated that
such materials may expand beyond the cell-body and be deposited as
trails. As discussed elsewhere (Moscona, 1960), the characteristics of
such cellular deposits appear to differ according to treatment of the
cells, variations in medium composition, pH, ions, temperature. In
general, the deposits are more noticeable in low-protein media (see also
Rosenberg, 1960; A. C. Taylor, 1961; L. Weiss, 1961), but whether this
is due to increase in quantity or to a changed colloidal state is unknown.
The possibility was considered that tracks of such extracellular material
might provide guide-lines for migrating cells, thus imposing on an
aggregating system a certain degree of non-randomness. The actual
deposition by migrating cells of tracks that were followed by other cells
was clearly visualized in time-lapse cine-films (Pomerat, 1959; unpublished films by this reviewer; similar effects are discussed by Gustafson,
1961). The question was raised whether the composition or the physicochemical properties of such extracellular materials might not provide
"information" transferable to cells coming in contact with it, and thus
play a role in "self-recognition" effects. Similar speculative possibilities
were recently considered by L. Weiss (1961). Okada (1959, 1960) found
that the colloidal material appearing in aggregating cell cultures took
up S
35 and that the label was incorporated in the matrix of the aggregates. Functional similarities may thus exist between the extra-cellular
materials of dissociated cells appearing in the course of aggregation and
the intercellular substances of restituted tissues (Moscona, 1960, 1962b).
As stated above, self-aggregation does not lend itself readily to
rigorous control; the results, complicated by secondary effects, are
variable and difficult to assess quantitatively. The use of hanging drops,
in which the cells sink to the interface between the liquid and the air
(Grover, 1962), reduces some of the difficulties but introduces others.
Self-aggregation is therefore useful primarily in descriptive work on
histogenesis of aggregates, and in studies on cell movement in relation
to various substrata or interfaces. For more precise studies on the
dynamics of cell contact and histotypic association, experimental
systems are to be preferred in which the cells are brought together
independently of their individual mobilities, under conditions amenable
to control and measurement.
C. MEDIATED AGGREGATION
1. Centrifugation
Trinkaus and Groves (1955) and Weiss and James (1955) used
centrifugation to make pellets of trypsin-dissociated cells which,